The present invention relates to a creping doctor for creping a soft paper web off a yankee cylinder.
U.S. Pat. No. 4.019.953 (corresponding to SE-B-381899) describes an apparatus for removing dust released in the form of fibers and other particles from a paper web when this is creped off a yankee cylinder, said apparatus comprising a pick-up receptacle disposed below the dust-generating region and at the side of a supporting beam. A duct for the supply of compressed air is connected to the receptacle as well as a suction pipe to remove the dust laden air entrained by jets of compressed air supplied at various points across the direction of movement of the web. Such an apparatus is clumsy, requires considerable space and is relatively ineffective. It does not take care of the dust released on the side of the doctor blade facing the envelope surface of the yankee cylinder. Moreover, there is a space between the pick-up receptacle and supporting beam of the creping doctor, through which dust can pass without being drawn into the pick-up receptacle. Since the apparatus requires a large space the replacement of the doctor blade is made difficult.
DE-A-1 104 319 describes a doctor especially for drying cylinders in paper machines. This doctor is provided with a doctor blade for cleaning the envelope surface of the drying cylinder by abutting with a low pressure against the envelope surface so that pulp fibers, fillers and the like are scraped off the surface. The doctor has a doctor holder mounted on a bracket on a pipe, a pressure plate abutting against the doctor blade and being provided with a plurality of suction openings communicating with the internal space of the pipe via a channel through the doctor holder and bracket. This known doctor cannot be used as a creping doctor, partly because it is not designed to withstand the considerable linear pressure of about 360-500 kg/m which must be exerted by a doctor blade on the envelope surface of a yankee cylinder, neither is the suction connection from said suction openings to the interior of the pipe arranged or adapted for the relatively large quantities of dust released when creping a paper web off a yankee cylinder. A similar doctor is described in DE-A-1 268 955, this being supplemented with compressed-air channels discharging in the vicinity of the doctor blade, and special closing covers to regulate the suction air along the pipe. This known doctor is not suitable as a creping doctor for yankee cylinders either, for the same reasons as discussed for the doctor according to DE-A-1 104 319.
A problem which has not been touched in greater detail in the patent literature, and which is specific to creping a paper web off a yankee cylinder, refers to the dust produced inside the doctor blade, i.e. on the side thereof facing the envelope surface of the yankee cylinder, and with which the doctor blade forms an acute angle. Dust in and from this internal dust-generating space has been found to form bundles of fibers which collect on the creping doctor and its supporting beam and which grow to such a size that they are pulled along by the yankee cylinder and adhere to its sticky envelope surface. If no cleaning doctor is provided downstream of the creping doctor, or if it does not temporarily function, these fiber bundles will accompany the yankee cylinder for one or two turns and be covered by the paper web to which they may adhere, thus causing a deterioration in the quality. There is also a risk of breakage of the paper web. Furthermore, there is considerable risk of this dust inside the doctor blade and said fiber bundles being ignited by sparks which may be generated on external parts of the yankee cylinder which are free from adhesive, due to the fact that the doctor blade made of metal presses with high linear pressure (about 360-500 kg/m) against the envelope surface of the yankee cylinder rotating at high speed. Also when replacing doctor blades there are problems with dust present between the envelope surface and, during this work, the slightly pivotally lowered creping doctor and its pivotable supporting beam, said dust being liberated as earlier but at a doctor located slightly upstream. A further problem is the great temperature differences prevailing on both sides of the supporting beam since the side of the supporting beam facing the hot yankee cylinder acquires a higher temperature than the opposite side. These temperature differences cause corresponding differences in linear expansion so that the supporting beam and creping doctor loose the desired straightness which in turn causes the doctor blade to press with an uneven linear pressure along the yankee cylinder so that the creping off effect ceases and there is risk of breakdown due to web breakage.